The deployment of heterogeneous cellular communication networks, which are referred to herein as Heterogeneous Networks (HetNets), is largely seen as one of the most cost efficient solutions to meeting the constantly increasing demand for higher data rates in the coming generations of cellular systems. Such deployments include several Low Power Nodes (LPNs) of diverse nature (e.g., micro, pico, and femto eNode Bs in the case of 3GPP Long Term Evolution (LTE)). These LPNs transform conventional homogeneous cellular communication network architecture into a fragmented multi-layered architecture.
HetNets are resistant to strains on signal power normally resulting from increasing distance from the transmitting point and are well known to defy the inverse square law of distance by moving base stations (BSs) (i.e., macro nodes and the LPNs) closer to user equipment devices (UEs) and providing similar Quality-of-Service (QoS) throughout the cell area. Thus, HetNet deployments possess an inherent capability to address the limitations implied by channel capacity and provide a uniform user experience throughout the cell area irrespective of user location. The potential of HetNets to bring gains in coverage and capacity are widely acknowledged. The major advantages or benefits can be summarized as:                Moving the BSs closer to the UEs results in better radio link conditions which in turn leads to higher data rates for UEs connected to LPNs.        LPN cells provide access to the UEs previously handled by the macro layer, thus reducing the load from the macro cell (called macro offloading) which results in higher availability of resources and thus higher data rates for the users connected to macro nodes.        In general, HetNet deployments provide uniform data rates within a given area.        
However, even though there are significant advantages brought by HetNet architectures, there are a number of concerns to be addressed. For instance, the high number of parameters associated with LPNs, e.g., restrictions on transmission power, access rights, and backhaul capacities, has a direct impact on system performance and makes the selection of LPN type and supported features a highly complicated task. The decision depends mainly on the goal to be achieved with the addition of LPNs (e.g., capacity vs. data rate improvement, or both).
The co-existence of cells with different power levels in HetNets has several implications on system access and mobility procedures. In a macro-only deployment, the cell selection process for the UEs is generally based on the Reference Signal Received Power (RSRP), otherwise known as Received Signal Strength (RSS). This means that the UEs get attached to the cell from which they attain strongest RSS. However, employing this access procedure to HetNets can intensify the interference scenarios in the uplink and can further lead to load imbalance situations where most of the UEs get connected to macro cells while LPN cells are underutilized. In LTE, the power difference between the macro and femto cells is about 23 decibel-milliwatts (dBm). This means that UEs that have a lower path loss to the LPN cell still receive high RSRP from the macro node and therefore are connected to the macro node rather than the LPN. This causes high interference in the uplink, which results in an uneven distribution of UEs in the macro and LPN cell layer.
The aforementioned load imbalance issue has been a topic of several researches. One proposed solution is a concept of “Range Extension” which provides a simulated expansion in the range of the LPN when making a decision on UE association with the LPN. This means that whenever a UE is associating to a LPN, an artificial offset threshold is to be added to the actual RSRP value to be used for the cell association decision. In contrast, in the case of macro node, the association decisions are based on the actual received signal strength in most of the cases. The concept of Range Extension (RE) enables an optimal association of users throughout the coverage area, which leads to enhanced system performance and load reduction from the macro cell at the same time.
The drawback of range extension is that UEs located in the extended range of small cells and connected to LPNs might experience difficulties in correctly receiving downlink control information transmitted by the downlink. Specifically, for LTE, UEs located in the extended range of LPNs and connected to LPNs may experience difficulty in correctly receiving downlink control information in the Physical Downlink Control Channel (PDCCH) since those UEs are experiencing negative downlink geometry. To minimize the effect of high interference onto the PDCCH transmitted by the LPN, Almost Blank Subframes (ABSs) are used. During ABSs at the macro node, there is no data transmission from the macro node, which provides the advantage of low interference to LPN cells.
During an ABS at the macro node, the transmission from the macro node does not contain data or control information, but mainly Common, or Cell-Specific, Reference Signal (CRS). This means that the corresponding uplink subframe (i.e., for LTE, the uplink subframe that occurs 4 Transmit Time Intervals (TTIs) later) in the macro node will have no data transmission either because no Downlink Control Information (DCI) (or control information in general) was transmitted in downlink during the ABS. As a result, resources are not fully utilized leading to a decrease in capacity and spectral efficiency.